The use of dyes, inks and other chemicals in the marking of commercial, consumer and industrial goods places restrictions on supply chains, logistics and the environment. Processes that can mark without the use of dyes, inks or other chemicals can therefore provide a distinct advantage. Laser marking is also generally more versatile, reproducible, and can provide marks that have a higher quality and durability than chemical methods such as silk screens.
Laser marking has been applied to many materials including metals. Once perfected for a particular material, the laser marking process is typically reliable, repeatable, and amenable to high-throughput high-yield production. An example is the color marking of anodized aluminium, a material that is in widespread use as it is lightweight, strong, easily shaped, and has a durable surface finish. The anodized surface is generally dyed with colored dyes. However it is also possible to laser mark anodized aluminium without the use of dyes, inks or other chemicals. Laser light can be used directly to form various colors either within the anodization or in the interface region between the oxide layer that forms the anodization and the aluminium. Similar marks can also be made on other anodized metal surfaces such as titanium, zinc, magnesium, niobium and tantalum.
It is very desirable in consumer goods to have a mark that is distinctive in shape, quality and color, and have a high color contrast to the surrounding material. High quality black marks in anodized aluminium are highly desirable and commercially very important.
U.S. Pat. No. 6,777,098 describes a method of marking anodized aluminium articles with black marks which occur in a layer between the anodization and the aluminium and therefore are as durable as the anodized surface. The marks described therein are described as being dark grey or black in hue and somewhat less shiny than unmarked portion using nanosecond range infrared laser pulses. As taught in U.S. Pat. No. 8,451,873, making marks according to the methods claimed in U.S. Pat. No. 6,777,098 are disadvantageous because (i) creating commercially desirable black marks with nanosecond range pulses tends to cause destruction of the oxide layer, and (ii) cleaning of the aluminium following polishing or other processing adds another step in the process, with associated expense, and possibly disturbs a desired surface finish.
U.S. Pat. No. 8,451,873 discloses a method for creating a mark on an anodized specimen. The method involves providing a laser marking system having controllable laser pulse parameters, determining the laser pulse parameters associated with the desired properties, and directing the laser marking system to mark the article using the selected laser pulse parameters. Laser marks so made have an optical density that ranges from transparent to opaque, a white color, a texture indistinguishable from the surrounding article, and durable, substantially intact anodization. The patent teaches that marks created using laser pulses greater than 1 nanosecond results in clear signs of cracking of the anodization. In particular, the patent teaches that when marking with prior art nanosecond pulses, applying enough laser pulse energy to the surface to make dark marks causes damage to the anodization which causes the appearance of the marks to change with viewing angle. The patent also teaches solving this problem by using pulses having pulse widths of approximately 10 ps. Marks produced by using pulses having pulse widths of approximately 10 ps or less do not damage the anodization, regardless of how dark the marks are, and nor do the marks change in appearance with viewing angle. Such marks are typical of so-called “cold processing” that utilize multi-photon absorption effects in the material. Cold processing (such as cold ablation) does not rely on thermal effects to produce the desired processing effect, and therefore has little if any thermal damage surrounding the processed area. Cold processing relies on femtosecond lasers, or picosecond lasers having pulse widths up to around 10 ps to 50 ps. The marks, quantified by the CIE system of colorimetry, have a chromaticity less than L=40, a=5, and b=10. Although the picosecond lasers used in the patent were much less expensive than femtosecond lasers, the picosecond lasers users are more expensive than nanosecond lasers because they rely on very advanced techniques and components such as optical pulse compressors to produce the very narrow laser pulse widths. Moreover, an L value lower than approximately 30 is more commercially important, and for this, the picosecond lasers used do not write the marks quickly enough for many commercial applications where cost is at a premium. It is advantageous not to rely on expensive techniques or components such as optical pulse compression and optical pulse compressors.
The damage to the anodization layers caused by the use of nanosecond lasers is a particular problem. It is believed that this is caused by thermal effects, and heat build up below the anodization layer as consecutive pulses are written.
There is a need for a method for laser marking a metal surface with a desired color that avoids the aforementioned problems.